How do you keep the second battery against overcharging ?

[Red Dragon]

Do you have a smart charger dc/dc 12v ?
Or is connected as soon as the engine works and the second battery is not balanced.

[Burgerman]

Not sure I know what you are asking.

If the two batteries are connected together, 2nd battery and engine battery, directly, then both will charge as soon as the engine runs, neither will overcharge.

If they are separate, how are you charging your 2nd battery?

My van has 1 battery for everything.

[Red Dragon]

There are two batteries.
one is the car battery(engine)
the Second battery is the battery boost.
if the first in weak you Push the button
the relay will connect the 2 battery.
and now you can start the car.
-----------------------------------------------------------------
if you start the car and you have more than 13.0v
the second battery connect and now it is like one battery and the alternator charge and see one battery.
first battery is 12.5v (75AH) and the second 11.9V(40AH).

engine start you see one battery 14.4-13.8v.
Batteries will not be balanced.14.4v

I open the second battery it was dry I filled water
the acid was strong.(White and active)

http://highfields-arc.co.uk/vehicles/auxchg.htm

[Burgerman]

Lead batteries don't work like lithiums in your modified chair.

You can connect any number (1 to infinite number) of lead batteries together and charge them all at 14.4v.
You do this for a very long time and HOLD THEM at 14.4v for about 5 hours or to around 100th to 500th C. In non cyclic use this is considered charged. Until current (Amps) drop to a very low level. At this point both are fully charged. The batteries balance each other naturally.

Alternators never do this, unless you drive for 5 to 8 hours solidly... So the batteries are always between 80 to 95 percent charged in average daily use.

If you use a battery charger, and charge a battery to 14.4v and to better than 100thC and then allow it to sit on FLOAT for a further 5 hours or more it will read as follows if completely DISCONNECTED from your vehicle:

After 10 mins 13.6V approx.
After 1 day 12.8V to 13.2V depending on age, type, temp, condition etc. In all cases this is 100 PERCENT FULL!

If the same battery is connected to a car:
After 10 mins, 13.0V to 13.3V
After 1 day 12.5V to 12.8V depending on age, type, temp, condition etc. AND RESIDUAL CURRENT DRAIN dragging the voltage low artificially. In all cases this is Likely 95 to 100% full!

And there's absolutely no point in having 2 separate starter batteries. MUCH better to connect both together full time 24/7 as one big battery. You will get far more minutes of cranking time, and greater reserve time, due to much less peukert affect.

[Red Dragon]

this is how car that have ramp built.
one battery is 30% and the second is full
Alternators work 15-30 Minutes city drive

one batter do all the work it is 75HA ,alarm, Locating signal system, Ramp ,door and more.
and the Second battery(sos) is for starter boost if you need it!
if the starter battery is Weak ,empty.(if you do not drive the car for 2 week+ than you need help you use the Second battery for 1.5 sec)


Usually I don't use the Second battery it is smaller and always 100%++ full.(this is the Problem)
the second battery is in the rear the cable is + and 5 meter+ long.
one battery is 12.1v 75ah
the second is 12.8v 40ah
this is how it start the day

see how ctek can balanced
http://www.ctek.com/au/en/chargers/D250S%20DUAL
But I do not think he can Return 120AH to the engine battery starter.

[Burgerman]

You don't need that.

Your cars alternator can connect directly to both batteries. FULL TIME. Or just for charging. You can safely charge one FULL battery, and one dead empty battery in parallel. You cannot overcharge the 2nd full battery. Holding a full battery at 14.4v for a few hours will not hurt it.

Going OVER 14.4v may do. But that shouldn't happen. If this is happening, get a large 50A or more low voltage drop diode and add this to the charge circuit. It will drop .2v to .4v from the charge voltage.

The battery you think is discharged (engine battery) is a FALSE reading, and will always read lower because of the voltage depression caused by the vans standby current. You cannot tell the state of charge this way. To get an accurate reading from it you will need to disconnect the battery. Wait 24 hours. Then and only then read its voltage.

So forget about separating these batteries, and connect both permanently together. So both voltages are the same. You will get much better starting performance and both will charge correctly.

If you have 2x 50Ah batteries and use them separately, you may get for eg 6 mins of total cranking time. Connect them in parallel and you will get about 8 or 9 mins. And the starter battery will last longer, and will be less discharged in the same storage/parked time.

http://en.wikipedia.org/wiki/Schottky_diode

Much of this page is simply wrong!!! http://highfields-arc.co.uk/vehicles/auxchg.htm

[Burgerman]

FROM THAT PAGE.
(CONNECTED TOGETHER PERMENANTLY)

1 A higher capacity battery will tend to dis-charge into a lower capacity battery when charging is stopped (engine switched off) until both batteries are the same (low) capacity.
WRONG, BOTH WILL CHARGE TOGETHER AND WILL CLOSELY BALANCE EACH OTHER AT ALL TIMES. PROVIDED THE CABLES ARE A SENSIBLE SIZE.

2 Damage could occur to the lower capacity battery as a result of (1) if there is a large difference between the capacity of the 2 batteries.
RUBBISH, THEY BOTH CHARGE BASED ON VOLTAGE. SO WILL BOTH BE AT THE SAME % CHARGE AT ALL TIMES.

3 Should one of the batteries be unable to hold a charge then the other battery will be drained as well.
CORRECT. THROW AWAY BAD BATTERIES...

4 The total (standing) battery capacity will be 2 times the capacity of the lower capacity battery (the higher capacity battery not realising it's full potential) and, if a small battery was used for the axillary battery, may not be sufficient to restart the vehicle.
COMPLETELY WRONG. YOU CAN PARALLEL CONNECT 10AH AND 1000AH BATTERIES AND YOU GET 1010AH. THIS GUY IS CLUELESS!

5 If the charging circuit "sees" the low state of the higher capacity battery and tries to charge it up fully the lower capacity battery could "boil" (known as "gassing") because of over charging, damaging it and the vehicle (gassing emits sulfuric acid gas which is good at eating metal).
COMPLETELTY WRONG. BOTH BATTERIES SEE AND ARE AT THE SAME VOLTAGE AND STATE OF CHARGE AT ALL TIMES. AND BEHAVE AS 1 BIG BATTERY. AND NOTHING CAN BOIL AT 14.4V OR LESS IN ANY CASE...

6 Even if both batteries are the same make, model and capacity, as they age they will differ from each other, ending up with the problems listed above.
WRONG. OR ALL OUR WHEELCHAIR BATTERIES WOULD SCREW UP. IN PARALLEL IT PRETTY MUCH DOESENT MATTER IF ONE IS OLDER, DIFFERENT MAKE, DIFFERENT TYPE ETC. THIS GUY IS A RETARD.

7 Both batteries will become depleted after using the extra equipment with the engine stopped, hardly a good solution if you want to start the engine again.
BUT THEY WILL TAKE MORE THAN TWICE AS LONG TO DEPLETE... AND WILL START THE ENGINE WHEN VERY DEPLETED AS THERE WILL BE MUCH LESS VOLTAGE DROP WITH 2 BATTERIES UNDER STARTER LOAD, AND LESS PEUKERT EFFECT TOO.

[LROBBINS]

Consider the fact that every battery has multiple pairs of plates connected in parallel to form groups (cells) and then those cells are connected in series. Every two pairs of plates will be slightly different. If any of the nonsense in that article were true, no battery would work or last as every charge cycle would be destroying it. Two similar-chemistry batteries in parallel BEHAVE AS ONE BATTERY, period. Problems with over/under charge of weaker/stronger batteries happen when they are connected in series, not when they are in parallel, and even that can be mitigated by having the series batteries as equal as possible to start with (in the series case, you definitely DON'T want different AHr capacity batteries).
Ciao,
Lenny

[Burgerman]

Actually he seems to be confused by series/parallel connection on that page. Because if you ignore the parallel part, and presume he is talking about series connection then that list on NO's does make sense.